Heavy duty rock bits are employed for drilling wells in subterranean formations for oil, gas, geothermal steam and the like. Such bits have a body connected to a drill string comprising a plurality, typically three, of legs and a hollow cutter cone mounted on each leg for drilling rock formations. The cutter cones are mounted on steel journal pins integral with the lower end of each leg of the bit body.
The cutter cones are maintained on the journal pins and provided rotatable movement thereon by bearings. The bearings are lubricated with a special grease adapted to conditions encountered by the drill bit. The grease is prevented from leaking by an O-ring cone seal positioned at the base of the journal pin between the cutter cone and the juncture of the journal pin and the bit body. The cone seal also prevents foreign material from being introduced between the cutter cone and journal pin.
In use the drill string and bit body are rotated in the bore hole and each cone is caused to rotate on its respective journal pin as the cone contacts the bottom of the bore hole that is being drilled. High pressures and temperatures are encountered as such rock bits are used in hard, tough formations. The total useful life of a rock bit in such severe environments is on the order of 20 to 200 hours for bits in sizes of about 61/2 inch to about 121/4 inch diameter at depths of about 5,000 feet to about 20,000 feet. Useful lifetimes of about 65 hours to about 150 hours are typical.
When a rock bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string for replacing the bit. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. This time can become a significant portion of the total time for completing a well, particularly as the well depths become great. It is therefore desirable to maximize the lifetime of a drill bit in a rock formation. Prolonging the time of drilling minimizes the lost time in "round tripping" the drill string for replacing bits.
Replacement of a drill bit can be required for a number of reasons, including wearing out or breakage of the structure contacting the rock formation. One reason for replacing the rock bits includes failure or severe wear of the bearings on which the cutter cones are mounted. These bearings are subject to very high pressure drilling loads, high hydrostatic pressures in the hole being drilled, and high temperatures due to drilling, as well as elevated temperatures in the formation being drilled. Considerable development work has been conducted to produce bearing structures which employ materials that minimize wear and failure of such bearings.
Excessive wear and failure of bearings can also be caused by lubrication failures which can be attributed to misfit of bearings and cone seal failures, as well as problems with the lubricating grease.
It has recently been discovered that in certain geographical regions, particularly in the Midwest region of the United States, there is an abnormally high failure rate of rock bits due to failure of the cone seals. Failure of the cone seals allows lubricating grease to escape and permits drilling mud or the like to enter the bearings. Such materials are abrasive and quickly damage the bearings. Neither the cause of the cone seal failures nor a solution to the problem has heretofore been established.